Frequency shift keying



Dec. 13, 1949 w. A. MILLER FREQUENCY SHIFT KEYING 2 Sheet-Sheet 1 FiledMay 3, 1945 INVENTOR #244? A Muff. BY )1 g/ m ATTORNEY Patented Dec. 13,1949 FREQUENCY SHIFT KEYING William A. Miller, Port Jefferson, N; Y.,assignor to Radio Corporation of America, a corporation or DelawareApplication May 3, 1945, Serial No. 591,730

(Cl. 178,-y-66) '7 Claims.

This application relates to improved methods of and means for signallingby telegraphy. The application in particular discloses an improvedtelegraphy system of the frequency shift type, wherein a carrier iskeyed from a first frequency which may represent mark to a secondfrequency, separated from the first frequency as desired, which mayrepresent space.

Frequency shift telegraphy systems are known in the prior art. Onecommon method of frequency shift keying involves keying the phase ofoscillations from a crystal controlled oscillator and multiplying thephase modulated currents the amount necessary to get the desiredfrequency difference between marking frequency and spacing frequency.This system has the advantage of using crystal controlled carrierenergy, but the amount of frequency shift obtainable is dependent on thenumber of multipliers used and the amount of frequency shift is notreadily changed.

An object of my invention is to provide a frequency shift telegraphysystem wherein the carrier, energy is derived from crystal controlledgenerators and wherein the amount of frequency shift may be readilychanged. In the system of my invention two crystal controlledoscillators, one operating at mark frequency and the other at spacefrequency are alternatively turned on and off by the keying signals. Inmy system the frequency shift can easily be made the desired amountsubject only to the band pass capability of the stages in thetransmitter following the modulator, and this without recourse tofrequency multiplication. All that is required is the retuning of thecrystal controlled generators or replacement of the crystal in one orboth there-of.

Another commonly known method of frequency shift keying involves anunstable oscillator which is keyed in accordance with telegraphy signalsfrom a first frequency which may represent space to a second frequencywhich may represent mark. The modulated oscillations are then referredto, i. e., mixed with oscillations of fixed frequency developed, forexample, in a crystal controlled oscillator. A beat note is selected andused for signalling purposes. In systems of this type the frequencystability may be poor, particularly since there is generally noinsurance that a stopping of the crystals controlled comparisonoscillator will stop the transmission, and then the mean frequency maydrift radically.

An additional object of my invention is to provide an improved frequencyshift telegraphy system wherein the signalling energy is developed incrystal controlled oscillators and wherein the stability of the systemis good at all times.

The two systems known in the prior art as described broadly hereinbeforeinclude as stated in one case frequency multipliers and in the other anunstable keyed generator, a crystal oscillator,

elements of the system of converting circuits, etc., and both aresomewhat complicated and use considerable equipment. An object of thepresent invention is to provide a method of and means for producingfrequency shift oscillatory energy the frequency of which is stabilizedand yet may be shifted, which means is simple in nature, eificient inoperation, and uses a minimum amount of equipment. In the known methodof frequency shift signalling involving phase modulation of theoscillations generated and multiplication of the phase modulatedoscillations, the amount of frequency or phase shift of the output of acrystal controlled oscillation generator obtainable is very small unlessa poor crystal is used in the oscillation generator. Theamount by whicha crystal generator frequency can be shifted is nearly a constant pencentage of the frequency of oscillation andthe'refo-re frequency shiftkeying by pulling the crystal controlled generator frequency ispractically impossible in the low frequency and medium frequency ranges.In my improved system I provide a pair of crystal oscillators operatingat different frequencies and means for turning the same on and offalternately to generate mark and space frequencies and improved meansfor supplying the generated oscillations to a common amplifier stage.The difierence frequency may be changed merely by changing a crystal orboth crystals desired.

In describing my invention in detail, reference will be made to theattached drawings wherein Fig. 1 illustrates by circuit connection theessential features of a frequency shift telegraphy sys tem arranged inaccordance with my inv and operated to carry out my method. illustratesa wave forming network for use 21. input of the keyer of Fig. 1. Fig. 2l ust graphically the characteristics of certain 1; while Fl 3b, 3c,36., 3e, 3j, and 3c illustrate graphla.-. operation of the system ofFig. 1.

In Fig. 1 the tubes Vi and V2 are interconnected to form a tripping ortriggering circuit. Tubes V4 and V5 are each connected in a crystalcontrolled generator circuit, including crystals XI and X2 respectively,one operating at frequency It and the other operating at frequency f2.Tube V3 is a buffer and amplifier stage to which the frequency shiftedtelegraphy signals developed in the prior mentioned tubes arealternately amplified and fed to the following stages not shown, butwhich may include power amplifier stages, etc.

Tubes V! and V2 have their anodes and control grids 6 and Bcross-coupled by resistors if? and I2, and their anodes coupled to thepositive terminal of a source of direct current potentials by resistorsI l and it which may be shunted by condensers Cl and 02 respectively.The tube VI .5 has its cathode li coupled to a point of high radiofrequency potential on tank circuit Ti and through a portion of the tankcircuit to ground and thence to the negative terminal of the source ofdirect current potential mentioned above. The cathode l9 of tube V2 issimilarly connected through a portion of tank T2 to ground.

The control grid 5 of tube Vi is connected by resistor l8 to thenegative terminal of a bias source BC, a positive terminal of which isgrounded While the control grid 8 of tube V2 is connected to source BCby resistor 25. Resistor it may be shunted by a condenser Cl, Whileresistor may be shunted by a condenser C2. The circuit described hereinis somewhat of the nature of the triggering circuit described in FinchU. S. Patent #1,844,950, issued Feb. 15, 1932, and in particular asdescribed in my U. S. application Serial No. 688,742, filed August 6,1946.

The tube V4 has its anode, cathode and control grid regenerativelycoupled by tank circuit Ti and the crystal Xi operating at a frequencyfl. The tube V5 has its electrodes coupled in a similar regenerativeoscillation generating circuit including tank circuit T2 and a crystaloperating at a frequency f2. The anode circuit of tube V 3 includes aparallel stuned circuit 22 coupling the anode to the positive terminalof a direct current source. The circuit 22 is parallel resonant at thefrequency of the oscillations generated in tube V4 and at the frequencyof the crystal Xi, and is of a Q discussed hereinafter. The circuit 24connected with the anode of tube V5 and to the positive terminal of thesource is parallel tuned to the frequency of the tank circuit T2 andcrystal X2, and is also of a Q which will be discussed hereinafter. Theanode of the tube V is coupled by a condenser 26 over a series tunedcircuit 23 and through a parallel tuned circuit 39 and over a resistance36 to the control 4 grid 35 of a coupling and amplifying tube V3. Theanode of the tube V5 is similarly coupled by a condenser 32, seriestuned circuit 40, and parallel tuned circuit 4'3 to the control grid oftube V3. Circuit 28 is series tuned to the frequency f2 at which thetank circuit T2 operates, while circuit 39 which is a stopping circuitis parallel tuned to the same frequency. Circuits it and G2 are seriesand parallel tuned respectively to the frequency fl at which the tankcircuit Tl operates. Preferably circuits 2B and 38, and @El and 42, aresharpl resonant at the requency to which they are tuned.

In practice the trigger circuit is so set that the current will be cutoff in one tube, sa V5, and 5;.

switched through the other tube V2. In an embodiment the negative biasapplied to the control grids 5 and B of tubes VI and V2 is made slightlygreater than cutoff. Then the values of resistances l 8, l2, l and itare so proportioned that tube Vl always assumes the nonconducting state,i. e., is initially cut off on the application of direct currentpotentials to the tubes electrodes. In these circuits, as is known, ifcurrent flow is initiated in one tube due to the crossconnectionsbetween the anodes and control grids the current is all switched to thistube while the other tube is cut off. In my system the adjustment issuch that tube V! is out off and the current in the trigger system isswitched through the tube V2. When tube Vl is cut off there is noloading on the tank circuit Tl of the oscillator tube V4 and thisoscillator Will operate to excite the grid of tube V3 by Way of thecoupling 25 with oscillatory energy at the frequenc fl. fl

';- at frequency f2.

will be determined by the resonant frequency of the crystal Xi betweenthe grid and anode of tube V4. The Q of the resonant circuit 22connected to the plate of tube Vt is relatively unimportant except thatit must be such that the response is wider than the band of frequenciesobtainable from the crystal controlled oscillator V5. The characteristicof this circuit designated Ql (fl) is shown in Fig. 2. A similarparallel tuned circuit 2 3 having similar characteristics with respect:to the frequency f2 is connected with the anode of tube V5. Thiscircuits resonance characteristic is also illustrated in Fig. 2 anddesignated Q2 (f2). The series resonant circuit 28 in the lead betweencoupling condenser 26 and the grid of tube V3 must not be too sharplytuned, otherwise it may present too much shunting effect at the grid oftube V3, although this in part is prevented by the parallel tunedcircuits in the same line. In many cases these two circuits will not benecessary. In other cases where feedback through the cathodes of tubesVl and/or V2 ma hamper the keying action these circuits will benecessary. The same remarks apply to the circuits Gil and 42 in theconnection between the anode of tube V5 and the grid of tube V3. Theanode 5-9 of tube V3 is coupled to the positive terminal of a source ofdirect current by a circuit 52 parallel resonant to a frequencyintermediate the frequencies fl and f2, but this circuit is sufiicientlybroad to include or pass both frequencies fl and f2. The resonantcharacteristic of this circuit is shown in Fig. 2 and designated Q3 (f3)When the first pulse of the keying signal, which, for example, may beformed by differentiation the character, is applied to the leads 2!,Fig. 1, the triggering circuit will operate to switch the current fromtube V2 through Vl so that now V2 will be cut off and VI will remain onuntil the next keying impulse. The triggering circuit is responsive toboth signs of pulse energy by coupling both grids to the keying input.For example, assume as stated above that rtube Vl is cut off and tube V2is on. Then if a positive pulse is applied at the lead 2! the grids ofboth tubes tend to become positive but the current is already atsaturation in tube V2. However, in tube Vi current is initiated and thedrop of potential through resistance is is applied to the control grid 5of tube V2 to cut down or reduce the current therethrough, and thisaction is cumulative because when the current through the tube V 2 isreduced the potential at the anode of tube V2 and at the grid of VIrises to increase the current through tube Vl. l/Vhen tube Vl is turnedon, i. e., becomes conductive and tube V2 is turned off, i. e., becomesnon-conductive by the first keying impulse tube V i stops oscillating "lat the frequenc fl because Vl is of low impedance and is connectedacross part of its tank circuit. In the meantime, tube V2 has become ofhigh impedance and the load across tank circuit T2 is reduced and tubeV5 starts oscillating Oscillations of the frequency 72 are now suppliedto the grid 39 of tube V3 and tube V5 continues to oscillate and excitethe grid of tube V3 until the next impulse is applied at lead 2 l,signifying the end of the signal character.

In the example assumed hereinbefore this signal pulse would be negativeand would not affect the flow of current through the tube V2 already cutcs", but would reduce the current flow through tube V! so that thetripping action would again take place, flipping the current through thetube V2 and cutting off the current in the tube VI. Then theoscillations would be stopped in tube V5, started in tube V4, and thegrid of tube V3 would be excited by oscillations Of frequency fl until achange in the pulse applied at lead "2! takes place.

The differentiating circuit may be as illustrated in Fig. la. The signalsuch as, for example, represented at the input lead is applied to thegrid 10 of a tube 52 having a load resistance RL connecting the tubesanode to a source of potential E the anode being coupled by a condenserC5 to the grid coupling condensers 9 and H over a time constant elementin the form of a resistance R. R, RL and C5 are small so that the timeconstant of the network is small as compared to the duration of thekeying pulses. When the signal appears on the grid if! the potentialswings negative and the anode potential rises, to a value approximatingE rapidly and charges condenser C5. A potential peak (positive) is builtup across the condenser C5 and applied to the trigger circuit throughcondensers 9 and l I. This potential trips the current in the triggercircuit and falls rapidly because condenser 05 discharges rapidly. Atthe end of the signal pulse the potential on grid if! swings toward zeropotential, the tube draws current and its anode potential drops rapidly. Condenser C5 is charged negative (in the other direction) and anegative potential is built up across the condenser C5 and applied tothe trigger tube grids through condensers 5 and II. This potentialswitches the current in the tubes of the trigger circuit and fallsrapidly because condenser C5 is small. The cycle then repeats as thesignal comes in. The grids s and 3 of the trigger tubes are excited bypotentials represented by Fib. 3b.

In the description of Fig. 1a it is assumed that condensers 9 and l lare coupling condensers appropriate for the keying rate used and thatresistances I 8 and 2s serve only as biasing elements. In many casescondensers 9 and H, and resistances l3 and 28 may be dimensioned tooperate also as time constant elements. Then, for example, the signalapplied at the input may be as shown adjacent the anode lead from thetube 12 in Fig. 1a. In other words, in practice resistances I8 and 2d ofFig. 1 may each also have the purpose of resistance R of Fig. 1a, andcondensers 9 and II of Fig. 1 may each have the purpose of condenser C5of Fig. 1a.

In Fig. 3a I have represented signal characters. In Fig. 32) I haverepresented keying signals by pulses of opposed polarity but whichpolarity (relative) is unimportant because as stated above thesepotentials are applied to the control grids 5 and 8 of both tubes V! andV2, and irrespective of their polarity initiate a reversal or switchingof the currents through the tubes when the same are applied. Supposethat on initiation of the signal character Fig. 3a the pulse applied atlead 2|, Fig. 1, is of one polarity, as shown, Fig. 31), while on thetermination of the signal character Fig. 3a the pulse applied at lead 2!is of opposed polarity as shown in Fig. 3b. The anode voltages of tubesVI and V2 are illustrated in Figs. 3c and 3d. Pulses of oscillations offrequency fl out of the tube V4 supplied to tube V3 are illustrated inFig. 3e, while the pulses of oscillations out of tube V5 applied to thegrid of tube V3 are illustrated in Fig. 3]. The composite outputincluding the alternatively keyed oscillations such as appears on theanode of tube V3 is shown in Fig. 39. It will be understood that thepolarity of the signals at the input may be reversed without al- 6tering the operativeness of the system or the principle of operationthereof. Reversing the polarity of the signals does not reverse theoutput from the trigger circuit.

The circuits 23 and 3D prevent oscillations of the frequency if fromreaching the oscillator including tube V4, while the circuits 45 and d2prevent oscillations of the frequency ,fl from reaching the circuits ofthe oscillator including tube V5.

The outputs are added as stated above in the anode circuit of tube V3,with the result that the character (mark) and interval (space) arerepresented by the frequencies f2 and fl respectively; thus frequencyshift keying has been accomplished.

The Q (Q3) of the tuned circuit 52 in the anode of tube V3 must be smallenough to allow both fl and f2 to pass. This characteristic has beenillustrated in Fig. 2. If extreme differences of frequency are to beused Q3 may be further decreased by the insertion of a resistor 56 inparallel to the inductance and capacity of circuit 52. This isaccomplished by closing the switch SI in Fig. 1, and is shown in aquantitative fashion by the characteristics in Fig. 2.

Any difficulties such as flashover, excess power dissipation, etc.,resulting from the fact that transient currents are set up in thecircuits when the frequency of operation is suddenly shifted between twovalues are overcome in a large measure by addition of the smallcondensers Cl and C2 in shunt to resistances IA and it and/or additionof the small condensers Cl and C2 in shunt to the grid to cathodes ofthe tubes V! and V2. These condensers are small and adjustable and whenplaced as shown will slow down the changeover of the trigger circuit.Thus the loading will be gradually taken off of the tube V5 oscillatorcircuit and gradually applied to the tube V4 oscillator circuit at thebeginning of a character and vice versa at the end of a character. Thisresults in a damping action on both frequencies 1! and f2 at the pointof changeover, so that the power present at the changeover (which wouldappear as transient energy) is much reduced over that present at themark or space operation. This is illustrated by the curves of Figs. 3cand 3d, 3e, 3 but no attempt to show the same to scale is made heresince this operation can be performed in such a short time that very fewcycles of either fl or f2 will be damped.

What is claimed is:

1. In a signalling system, a first oscillatory circuit operating at afirst frequency, a second oscillatory circuit operating at a secondfrequency, an output circuit coupled to said oscillatory circuits, apair of electron discharge tubes having input and output electrodes,leads including the impedance between electrodes of one of said tubes inone of said oscillatory circuits, leads including the impedance betweenelectrodes of the other of said tubes in the other of said oscillatorycircuits, and means for biasing one of said tubes to cutoff in thepresence of energy of one intensity, and for biasing the other of saidtubes to cutoff in the presence of energy of a second and differentintensity, the arrangement being such that the two oscillatory circuitsare rendered operative and inoperative alternately as the tubeconductivities are changed by the biases applied by said last namedmeans.

2. In a signalling system, an electron discharge device havingelectrodes regeneratively coupled by a circuit, including reactance, forthe produc-..

tion of oscillatory energy, a pair of electron discharge tubes eachhaving an anode, a cathode and a control grid, impedances cross-couplingthe anodes and control grids of said tubes, and a source of directcurrent connected by impeders between the anode and cathode of each tubethe arrangement being such that when current flow is started through onetube the other tube is biased to cutofi and vice versa, leads couplingthe impedance between electrodes of one of said tubes in shunt to aportion at least of the reactance of said first circuit, and means forapplying a control potential which varies between a positive value and anegative value to the input electrodes of said tubes, to render thetubes alternatively conductive and non-conductive, the arrangement beingsuch that when said one of said tubes is conductive its impedance is lowand being in shunt to a portion of the reactance of the first circuitloads said circuit to stop generation of oscillatory energy.

3. In a frequenc shift signalling system, a first oscillatory circuitoperating at a first frequency, a second oscillatory circuit operatingat a second frequency, an output circuit coupled to said oscillatorycircuits, a pair of electron discharge tubes having input and outputelectrodes, impedances cross-coupling the input and output electrodes ofsaid tubes, leads connecting the impedance between electrodes of one ofsaid tubes in shunt to a portion at least of one of said oscillatorycircuits, leads connecting the impedance between electrodes of the otherof said tubes in shunt to a portion at least of the other of saidoscillatory circuits, and means for applying energy the intensity ofwhich changes between two values in accordance with signals to the inputelectrodes of said tubes, to render the tubes alternatively conductiveand thereby alternatively load and unload said oscillatory circuits tomake the same alternatively operative and inoperative.

l. In a telegraph signalling system, a first oscilation generator havinga first generating circuit operating at a first frequency, a secondoscillation generator having a second generating circuit operating at asecond frequency, a combining circuit coupled to both of saidgenerators, a pair of electron discharge tubes having input and outputelectrodes, leads coupling the impedance between the output electrodesof one of said tubes in shunt to one of said generating circuits, leadscoupling the impedance between the output electrodes of the other ofsaid tubes in shunt to the other of said generating circuits, thearrangement being such that when a tube is conductive the load itintroduces in the oscillation generator with which it is in shunt stopsoperation thereof, means for applying signalling energy the intensity ofwhich varies in accordance with signals to the input electrodes of saidtubes, and means coupling said tubes in a triggering circuit such thatwhen the signal level changes one tube becomes conductive and the othertube is cut off, and when the signal level again changes the said onetube is cut off and the other tube becomes conductive.

5. In a telegraph signalling system, a first oscillation generatorhaving a first generating circuit operating at a first frequency, asecond oscillation generator having a second generating circuitoperating at a second frequency, an output circuit coupled to both ofsaid generators, a

circuit parallel tuned to the frequency of oper} ation of one generatorin the coupling between the other generator and the output circuit, acircuit parallel tuned to the frequency of operation of said othergenerator in the coupling between said one generator and the outputcircuit, a pair of electron discharge tubes having input and outputelectrodes, leads coupling the impedance between electrodes of one ofsaid tubes in shunt to one of said generating circuits, leads couplingthe impedance between the output electrodes of the other of said tubesin shunt to the other of said generating circuits, the arrangement beingsuch that when a tube is conductive the load it introduces in theoscillation generating circuit with which it is in shunt stops operationof said generator, means for applying signalling energy the intensity ofwhich varies in accordance with signals to the input electrodes of saidtubes, and means coupling said tubes in a triggering circuit such thatwhen the signal level changes one tube becomes conductive and the othertube is cut off, and when the signal level again changes the said onetube is cut off and the other tube becomes conductive.

6. In a signaling system, a first oscillation generator having a firstoscillation generating circuit operating at a first frequency, a secondoscillation generator having a second oscillation generating circuitoperating at a second frequency, a combining circuit coupled to both ofsaid generators, a pair of electron discharge tubes having input andoutput electrodes, leads coupling the impedance between the outputelectrodes of one of said tubes in shunt to one of said generatingcircuits, leads coupling the impedance between the output electrodes ofthe other of said tubes in shunt to the other of said oscillationgenerating circuits, the arrangement being such that when a tube isconductive the load it introduces in the oscillation generating circuitwith which it is in shunt stops operation of said generator, means forapplying modulating potentials which vary in accordance with signals tothe input electrodes of said tubes, means for biasing one of said tubesto out off when the signal potential level changes and means for biasingthe other of said tubes to cut off when the signal potential level againchanges.

7. A signaling system as recited in claim 1 including an output circuitcoupled to both of said oscillatory circuits, means in the couplingbetween said output circuit and said first oscillatory circuit forpreventing oscillatory energy of said second frequency from reachingsaid first oscillatory circuit and means in the coupling between saidoutput circuit and said second oscillatory circuit for preventingoscillatory energy of said first frequency from reaching said secondoscillatory circuit.

WILLIAM A. MILLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,033,948 Lowell Mar. 17, 19362,111,567 Lowell Mar. 22, 1938 2,118,917 Finch May 31, 1938 2,438,492Bascom Mar. 30, 1948

